Especially in aeronautical and aerospace engineering so-called structural parts or integral parts are used which comprise ribs extending parallel with respect to each other (mostly on one side, but in certain circumstances also on both sides), while the side that is not provided with ribs is plane. If ribs are present both in the longitudinal direction as well as the transversal direction of the component which extend approximately perpendicular with respect to one another, the component is provided with a cassette structure. In order to curve such components it is necessary to apply complex processes because the ribs, especially when they extend parallel to the direction of curvature, offer considerable resistance against shaping.
Shaping methods of the kind mentioned above have long been used in aeronautical and aerospace engineering for curving large-surface components such as airfoils or fuselage shells. Blasting shot with a particle diameter of up to 2 to 4 mm is used in the shaping of structural parts. Whereas the blasting shot is applied with the help of spinner gates for the large-surface machining of components, hand blasting units are used for locally limited shaping. Said hand blasting units are also used for curving ribs. In order to enable the purposeful shaping of ribs that are usually flat on the basis of the blasting geometry and blasting diameter, the ribs are partly covered with a mask so that the desired elongation gradient is achieved in the rib zones to be shaped. Rubber or another impact-absorbent material is used for covering the surface sections of the ribs that are not to be blasted. The coverage of the ribs is cumbersome, especially when several masks need to be produced.
As an alternative to the aforementioned shot blasting method, the so-called clamping method (Eckhold method) is known from the state of the art. In this method clamps grasp the rib with a kind of grasp with two spaced clamping jaws at two adjacent places. As a result of a short movement of the two clamping jaws away from each other or towards each other the rib is either locally extended or swaged. As a result of a repeated application along the longitudinal extension of the rib it is possible to produce convex or concave curvatures. The curvature can be influenced by the stroke of the clamp and the number of repetitions of said applications.
Even if such clamping methods can be automated, it is still disadvantageous due to the low extensions per stroke of the clamping jaws which result in a lengthy shaping process. Despite the principally possible automation, the performing of the clamping method requires much experience by the operator, especially due to the danger of buckling and the spring-back behavior of the ribs.
Generally known are further so-called age creep forming methods for structural parts. The component is produced in a plane shape in this case by metal cutting, especially milling. Then the component is placed in a mold which has the external shape of the finished part. This shaping process usually takes several hours. A further disadvantage is that special molds need to be produced for each geometry. It is further necessary to determine the parameters, temperature, pressure and time for each part separately. Furthermore, the application of the age creep forming method is excluded for materials which are not suitable for the thermal treatment conducted thereby. A further difficulty is overextending the part in the mold by a certain amount in order to compensate the spring-back after the removal of the component from the mold in order to ensure the precise desired geometry of the part.
The state of the art also includes the shot peen forming method as known from the U.S. Pat. No. 4,329,862 for shot peen forming of plate-like parts, especially airfoil structures. It is not provided in this connection that the airfoil parts to be processed with the blasting shot are reinforced by ribs. The said US patent specification merely teaches that the part is stretched in a first step by blasting with blasting shot on either side and to curve it into another direction by blasting it with blasting shot on merely one side.
Finally, a method applied in practice for shaping structural parts consists of milling the same from solid material with the help of modern CNC milling machines. Apart from the considerable material input, this is merely possible for structures that are curved to an only very low extent. The costs incurred for raw material to be provided with a large thickness are considerable. That is why this method can be used in an economically viable manner only in a very few limited cases, especially where large-surface components are concerned. Moreover, there are strong back-spring effects in the finished part that are the result of the metal cutting process and impair its dimensional stability.